Before I had been including within the .htaccess AddHandler application/x-httpd-php5s .php .js .css using the purpose that with similar PHP I possibly could produce the rules for Cache and Compression. Things I did exactly was customize the auto_prepend_file within the php.ini file, pointing it to some PHP file and thru there look into the variable that's send through the browsers within the header $_SERVER["HTTP_IF_MODIFIED_SINCE"].

When the dates were different, I'd answer having a header('HTTP/1.1 200 OK') which would send the JS or CSS files compressed.
When the dates were exactly the same, i quickly would send a header('HTTP/1.1 304 Not Modified') and also the browser weren't required to download the JS or CSS files in asked.